SM8120A
White LED Driver IC
OVERVIEW
The SM8120A is a high efficiency step-up DC/DC converter. Due to high voltage CMOS process realizing
24V output supply as maximum value, white LED of 2–4 lights connected in series can be lighted. By connect-
ing in series, current variation among LED is eliminated. Current value sent to white LED can be set by exter-
nal resistors. In addition, brightness can also be adjusted by control to FB pin or CE pin.
FEATURES
I
I
PINOUT
(Top view)
I
I
I
I
I
I
I
I
I
I
I
I
Boost-up control using PFM
White LED of 2-4 lights (connected in series)
lighted
Output current value can be set by external resis-
tors (51
Ω
: 9.8mA, 33
Ω
: 15.2mA, 24
Ω
: 20.8mA)
Brightness adjustable by control to FB pin or CE
pin
Current variation among LED decreased by high
precision
High efficient drive by step-up model
Supply voltage range: 2.4 to 5.5V
Maximum output voltage: 24V
Quiescent current: 80
µ
A (typ)
Standby current: 1.0
µ
A (max)
R
ON
(Switching MOS-Tr): 2
Ω
(typ)
Maximum switching frequency: 500kHz (typ)
Output current detection accuracy:
±
2%
Small package: SOT23-5
VDD
1
5
CE
VSS
2
SW
3
4
FB
PACKAGE DIMENSIONS
(Unit: mm)
I
I
I
I
I
I
I
I
I
Cellular phone
Pager
Digital still camera
Handy terminal
PDAs
Portable games
White LED drive
LCD bias supply
Flash memory supply
+ 0.2
1.6
−
0.1
+ 0.2
2.8
−
0.3
1.9 ± 0.2
2.9 ± 0.2
0.4 ± 0.1
+ 0.1
0.15
−
0.05
0.12 M
1.1 ± 0.1
0 ~ 0.10
0.8 ± 0.1
0.95
0.1
ORDERING INFORMATION
Device
SM8120AH
Package
SOT23-5
NIPPON PRECISION CIRCUITS INC.—1
0.20MIN
APPLICATIONS
SM8120A
BLOCK DIAGRAM
SW
VDD
FB
Buff
Q
R
VREF
OSC
CE
SOFT
START
S
COMP
VSS
PIN DESCRIPTION
Number
1
2
3
4
5
Name
VDD
VSS
SW
FB
CE
I/O
–
–
O
I
Ip
1
Power supply
GND
Coil switching
Feed back (Output current detection)
Chip enable (High active)
Description
1. Input with built-in pull-down resistor
NIPPON PRECISION CIRCUITS INC.—2
SM8120A
SPECIFICATIONS
Absolute Maximum Ratings
Parameter
Supply voltage range
Input voltage range
SW output voltage range
SW input current
Power dissipation
Operating temperature range
Storage temperature range
Symbol
V
DD
V
IN
V
SW
I
SW
P
D
T
opr
T
stg
Rating
−
0.3 to 6.5
V
SS
– 0.3 to V
DD
+ 0.3
–0.3 to 27
250
250 (Ta = 25
°
C)
–40 to 85
−
55 to 125
Unit
V
V
V
mA
mW
°
C
°
C
Electrical Characteristics
V
DD
= 3.6V, V
SS
= 0V, Ta = 25
°
C unless otherwise noted
Rating
Parameter
Supply voltage
Standby current
Quiescent current
SW-Tr ON resister
SW-Tr leak current
Maximum switching frequency
Duty
Input voltage
Pin
VDD
VDD
VDD
SW
SW
SW
SW
CE
CE
Input current
FB
Soft-start time
FB voltage
SW
FB
Symbol
V
DD
I
STB
I
DD
R
ON
I
LEAK
f
OSC
Duty
V
IH
V
IL
I
CE
I
FB
T
SS
V
FB
V
CE
= 3.6V
V
FB
= 0.5V
V
CE
= 0V
V
FB
= 1.0V
I
SW
= 100mA, V
DD
= 3.6V
V
SW
= V
DD
V
FB
= 0V
V
FB
= 0V
Condition
min
2.4
–
–
–
–
450
53
2.0
–
–
–1.0
–
0.49
typ
3.6
–
80
2.0
–
500
60
–
–
5.0
–
500
0.50
max
5.5
1.0
120
3.0
1.0
550
67
–
0.6
10
1.0
–
0.51
V
µA
µA
Ω
µA
kHz
%
V
V
µA
µA
µs
V
Unit
NIPPON PRECISION CIRCUITS INC.—3
SM8120A
OPERATION OVERVIEW
L
22µH
V
IN
3.0 to 4.5V
SBD
C
IN
4.7µF
SW
LED
FB
VDD
Buff
Q
R
VREF
S
COMP
ZD
C
OUT
1.0µF
Enable
Disable
OSC
CE
VSS
SOFT
START
R1
The SM8120A basic structure is a step-up DC/DC converter. The booster control employs Pulse Frequency
Modulation (PFM) which controls the frequency (500kHz max.) at constant SW-Tr ON time (1.2
µ
s typ.). The
LED current is set by a current-setting resistor R1 connected between pins FB (with stable voltage of 0.5V
typ.) and VSS.
When the switching transistor SW-Tr is ON, energy is stored in the inductor L. When SW-Tr is rapidly
switched OFF, the energy stored in the inductor generates a voltage across the terminals of the inductor. The
induced voltage, after being added to the input voltage, turns ON the Schottky barrier diode SBD and the
stored energy is transferred to the output capacitor. This sequence of events continues repeatedly, boosting the
output voltage.
The SM8120A features a built-in soft-start function. The soft-start time is approximately 500µs from after the
chip enable input CE rising edge. During this interval, the maximum SW-Tr ON time is restricted to 0.6
µ
s.
Selecting the Current-setting Resistor (R1)
The SM8120A control stabilizes the voltage on pin FB (0.5V typ.). Hence, the current-setting resistor R1 con-
nected between FB and VSS sets the LED current I
LED
, where the resistance R1 is given by the following
equation.
R1
=
0.5 / I
LED
FB
V
FB
=0.5V
I
LED
=0.5/R1
R1=0.5/I
LED
NIPPON PRECISION CIRCUITS INC.—4
SM8120A
Selecting the Inductor (L)
The recommended inductance for use with the SM8120A is 22µH. The inductor DC resistance affects the
power efficiency, therefore a low DC resistance inductor is recommended. Note also that the peak inductor cur-
rent I
peak
should not exceed the inductor maximum current rating. In pulsed current mode control, the peak
inductor current I
peak
is given by the following equation.
I
peak
= (V
IN
×
T
ON
) / L
For example, if the input voltage V
IN
is 3.6V, the inductance L is 22µH, and the SW-Tr ON time T
ON
is 1.2
µ
s,
then the peak inductor current I
peak
is (3.6
×
1.2
×
10
-6
) / (2.2
×
10
-6
) = 0.2A = 200mA.
Selecting the Capacitors (C
IN
, C
OUT
)
The recommended capacitances for use with the SM8120A are 4.7µF ceramic input capacitor C
IN
and 1.0µF
tantalum output capacitor C
OUT
. The input capacitor ESR ratings affect the ripple voltage, therefore capacitors
with low ESR rating are recommended. When the output capacitor ESR ratings are too low, it affect the
response to the FB pin, therefore tantalum capacitors are recommended. The input capacitor should be
mounted close to the SM8120A IC. Note that the capacitor voltage ratings should be selected to provide suffi-
cient margin for the applied input and output voltages.
For example, if a lithium-ion battery (2.5 to 4.5V) is connected to the input and 3 white LEDs connected in
series at the output draw 20mA, then the maximum input voltage is 4.5V and the maximum output voltage is
(4.0V
×
3 LEDs) + 0.5V = 12.5V. Therefore, the input capacitor should have a voltage rating of 6V, and the
output capacitor should have a voltage rating of 16V.
Selecting the Rectifier Schottky Barrier Diode (SBD)
The rectifier schottky barrier diode forward-direction voltage drop affects the power efficiency, therefore a
Schottky barrier diode with low forward-direction voltage drop is recommended. Note that the diode should be
selected to provide sufficient margin for the rated current and reverse-direction withstand voltage.
Board Layout Notes
The following precautions should be followed for stable device operation.
I
I
I
I
The inductor L and Schottky barrier diode SBD should be connected close to the pin SW using thick, short
circuit wiring.
The input capacitor C
IN
should be mounted close to the IC.
The IC supply voltage V
DD
wiring and inductor supply wiring should be isolated, reducing any common
impedances.
The ground wiring should be connected at a single point, reducing any common impedances.
SBD
L
SW
V
IN
C
IN
C
OUT
LED
VDD
CE
VSS
FB
R1
NIPPON PRECISION CIRCUITS INC.—5
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